Collision Handling Using Chaining in DSA Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is collision handling in hashing?

Collision handling is the method used to resolve situations when two keys hash to the same index in a hash table.

beginner

Explain chaining in collision handling.

Chaining handles collisions by storing all elements that hash to the same index in a linked list at that index.

beginner

What data structure is commonly used in chaining to store collided elements?

A linked list is commonly used to store collided elements at the same hash index in chaining.

intermediate

How does chaining affect the average search time in a hash table?

Chaining keeps average search time close to O(1) if the load factor is low, but it can degrade to O(n) if many collisions occur.

intermediate

Why is chaining preferred over open addressing in some cases?

Chaining is preferred because it handles collisions gracefully without clustering and allows the hash table to store more elements than its size.

What happens in chaining when two keys hash to the same index?

A One key replaces the other

B The keys are stored in separate arrays

C The table size is doubled

D Both keys are stored in a linked list at that index

In chaining, collided keys are stored in a linked list at the same index.

Which data structure is used in chaining for collision handling?

A Linked list

B Queue

C Stack

D Array

Chaining uses linked lists to store multiple elements at the same hash index.

What is the worst-case time complexity for search in chaining?

A O(1)

B O(log n)

C O(n)

D O(n log n)

In the worst case, all elements hash to the same index, making search O(n).

How does chaining handle a hash table that is full?

A It cannot store more elements

B It stores collided elements in linked lists, allowing more elements than table size

C It resizes the table automatically

D It deletes old elements

Chaining allows storing more elements than the table size by using linked lists at each index.

Which of these is a disadvantage of chaining?

A Requires extra memory for pointers

B Cannot handle collisions

C Always slower than open addressing

D Does not allow duplicate keys

Chaining requires extra memory to store pointers in linked lists.

Describe how collision handling using chaining works in a hash table.

Think about what happens when two keys get the same index.

Explain the advantages and disadvantages of using chaining for collision handling.

Consider memory use and search speed.

Practice

1. What is the main idea behind collision handling using chaining in hash tables?

easy

A. Replace the existing element with the new one

B. Resize the hash table immediately on collision

C. Store collided elements in a linked list at the same index

D. Ignore the new element causing collision

Solution

1. Step 1: Understand collision in hash tables

   When two keys hash to the same index, a collision occurs.

2. Step 2: How chaining handles collision

   Chaining stores all collided elements in a linked list at that index, allowing multiple elements.

3. Final Answer:

   Store collided elements in a linked list at the same index -> Option C

4. Quick Check:

   Chaining = linked list at index [OK]

Hint: Chaining means linked list at one index for collisions [OK]

Common Mistakes:

• Thinking collision replaces old element
• Assuming resizing happens immediately on collision
• Ignoring collided elements

2. Which of the following is the correct way to declare a hash table with chaining in Java using LinkedList?

easy

A. LinkedList<Integer> table = new LinkedList<>();

B. LinkedList<Integer>[] table = new LinkedList[10];

C. LinkedList<Integer>[] table = new LinkedList<>[10];

D. LinkedList<Integer> table = new LinkedList[10];

Solution

1. Step 1: Understand array of linked lists declaration

   We need an array of LinkedList objects, so use LinkedList<Integer>[].

2. Step 2: Correct Java syntax for array of generic linked lists

   Java allows new LinkedList[10] but not new LinkedList<>[10], so LinkedList<Integer>[] table = new LinkedList[10]; is correct.

3. Final Answer:

   LinkedList<Integer>[] table = new LinkedList[10]; -> Option B

4. Quick Check:

   Array of LinkedList = LinkedList[] table [OK]

Hint: Array of LinkedList uses LinkedList<Type>[] [OK]

Common Mistakes:

• Trying to use new LinkedList<>[size]
• Declaring LinkedList without array
• Wrong generic syntax in array creation

3. Consider the following Java code snippet for a hash table with chaining:

import java.util.LinkedList;

public class HashTable {
    LinkedList<Integer>[] table;
    int size = 5;

    public HashTable() {
        table = new LinkedList[size];
        for (int i = 0; i < size; i++) {
            table[i] = new LinkedList<>();
        }
    }

    public void insert(int key) {
        int index = key % size;
        table[index].add(key);
    }

    public void printTable() {
        for (int i = 0; i < size; i++) {
            System.out.print(i + ": ");
            for (int val : table[i]) {
                System.out.print(val + " -> ");
            }
            System.out.println("null");
        }
    }

    public static void main(String[] args) {
        HashTable ht = new HashTable();
        ht.insert(7);
        ht.insert(12);
        ht.insert(17);
        ht.printTable();
    }
}

What is the output of this program?

medium

A. 0: null
1: null
2: null
3: 7 -> 12 -> 17 -> null
4: null

B. 0: 7 -> 12 -> 17 -> null
1: null
2: null
3: null
4: null

C. 0: 12 -> null
1: null
2: 7 -> 17 -> null
3: null
4: null

D. 0: null
1: null
2: 7 -> 12 -> 17 -> null
3: null
4: null

Solution

1. Step 1: Calculate index for each inserted key

   Size is 5, so index = key % 5. 7 % 5 = 2, 12 % 5 = 2, 17 % 5 = 2.

2. Step 2: Insert keys at index 2 and print table

   All keys go to table[2]. So index 2 has 7 -> 12 -> 17 -> null, others are empty.

3. Final Answer:

   0: null 1: null 2: 7 -> 12 -> 17 -> null 3: null 4: null -> Option D

4. Quick Check:

   Keys mod 5 = 2, all at index 2 [OK]

Hint: Calculate key % size to find index [OK]

Common Mistakes:

• Miscomputing modulo operation
• Assuming keys go to different indices
• Confusing index with key value

4. The following code snippet tries to insert keys into a hash table with chaining but throws a NullPointerException. What is the error?

import java.util.LinkedList;

public class HashTable {
    LinkedList<Integer>[] table = new LinkedList[5];

    public void insert(int key) {
        int index = key % 5;
        table[index].add(key);
    }

    public static void main(String[] args) {
        HashTable ht = new HashTable();
        ht.insert(10);
    }
}

medium

A. The array of LinkedLists is not initialized with LinkedList objects

B. The modulo operation is incorrect

C. The insert method should return a value

D. The LinkedList should be replaced with ArrayList

Solution

1. Step 1: Identify cause of NullPointerException

   The array 'table' is created but each element is null because LinkedList objects are not initialized.

2. Step 2: Fix by initializing each LinkedList in the array

   Before inserting, each table[i] must be assigned new LinkedList<>() to avoid null pointer.

3. Final Answer:

   The array of LinkedLists is not initialized with LinkedList objects -> Option A

4. Quick Check:

   NullPointerException = uninitialized LinkedList array [OK]

Hint: Initialize each LinkedList before use [OK]

Common Mistakes:

• Ignoring initialization of linked lists
• Blaming modulo operation
• Thinking insert needs return value

5. You want to implement a hash table with chaining that supports insert and search operations. Given the following partial code, fill in the missing search method to return true if the key exists, otherwise false.

import java.util.LinkedList;

public class HashTable {
    LinkedList<Integer>[] table;
    int size = 7;

    public HashTable() {
        table = new LinkedList[size];
        for (int i = 0; i < size; i++) {
            table[i] = new LinkedList<>();
        }
    }

    public void insert(int key) {
        int index = key % size;
        table[index].add(key);
    }

    // Fill this method
    public boolean search(int key) {
        // ???
    }
}

Which of the following is the correct implementation of the search method?

hard

A. int index = key % size;
return table[index].contains(key);

B. int index = key % size;
return table[index].get(0) == key;

C. for (LinkedList<Integer> list : table) {
if (list.contains(key)) return true;
}
return false;

D. int index = key % size;
return table[index].indexOf(key) == -1;

Solution

1. Step 1: Calculate index using modulo

   Use key % size to find the correct linked list index.

2. Step 2: Check if linked list at index contains the key

   Use contains() method on linked list to check presence of key.

3. Final Answer:

   int index = key % size; return table[index].contains(key); -> Option A

4. Quick Check:

   Search = contains at index [OK]

Hint: Search key in linked list at key % size [OK]

Common Mistakes:

• Checking only first element in list
• Searching all lists instead of one
• Using indexOf == -1 incorrectly